H→γγsearch and direct photon pair production differential cross section measurement at DO.pdf

1、At a hadron collider, diphoton (γγ) production allows detailed studies of the Standard Model (SM), as well as as searches for new phenomena, such as new heavy resonances, extra spatial dimensions or cascade decays of hea

2、vy new particles. Within the SM, continuum γγ+X production is characterized by a steeplyfalling γγ mass spectrum, on top of which a heavy resonance decaying into γγ can potentially be observed. In particular, this is con

3、sidered one of the most promising discovery channels for a SM Higgs boson at the LHC, despite the small branching ratio of BR (H →γγ) ～ 0.2％ for 110 ＜ MHiggs ＜ 140 GeV. At the Tevatron, the dominant SM Higgs boson produc

4、tion mechanism is gluon fusion, followed by associated production with a W or Z boson, and vector boson fusion. While the SM Higgs production rate at the Tevatron is not sufficient to observe it in the γγ mode, the Hgg a

5、nd Hγγ couplings, being loop-mediated, are particularly sensitive to new physics effects. Furthermore, in some models beyond the SM, for instance, fermiophobic Higgs, with no couplings to fermions, the BR (H → γγ) can be

6、 enhanced significantly relative to the SM prediction, while has the SM-like production cross sections except the gluon fusion is absent.In this thesis, we present a search for a light Higgs boson in the diphoton final s

7、tate using 4.2 ± 0.3 fb-1 of the Dφ Run Ⅱ data, collected at the Fermilab Tevatron collider from April 2002 to December 2008. Good agreement between the data and the SM background prediction is observed. Since there is n

8、o evidence for new physics, we set 95％ C.L. limits on the production cross section times the branching ratio (σ × BR(H → γγ)) relative to the SM-like Higgs prediction for different assumed Higgs masses. The observed limi

9、ts (σ(limit)/σ(SM)) range from 11.9 to 35.2 for Higgs masses from 100 to 150 GeV, while the expected limits range from 17.5 to 32.0. This search is also interpreted in the context of the particular fermiophobic Higgs mod

10、el. The corresponding results have reached the same sensitivity as a single LEP experiement, setting a lower limit on the fermiophobic Higgs of Mbf ＞ 102.5 GeV (Mhf ＞ 107.5 GeV expected). We are slightly below the combin

11、ed LEP limit (Mhf ＞ 109.7 GeV). We also provide access to the Mhf ＞ 125 GeV region which was inaccessible at LEP.During the study, we found the major and irreducible background direct γγ (DPP) production is not well mode

12、lled by the current theoretical predictions: RESBOS, DIPHOX or PYTHIA. There is ～20％ theoretical uncertainty for the predicted values. Thus,for our Higgs search, we use the side-band fitting method to estimate DPP contri

13、bution directly from the data events. Furthermore, DPP production is also a significant background in searches for new phenomena, such as new heavy resonances, extra spatial dimensions, or cascade decays of heavy new par

14、ticles. Thus,precise measurements of the DPP cross sections for various kinematic variables and their theoretical understanding are extremely important for future Higgs and new phenomena searches.In this thesis, we also

15、present a precise measurement of the DPP single differential cross sections as a function of the diphoton mass, the transverse momentum of the diphoton system, the azimuthal angle between the photons, and the polar scatt

16、ering angle of the photons, as well as the double differential cross sections considering the last three kinematic variables in three diphoton mass bins, using 4.2 fb-1 data. These results are the first of their kind at